Field of the Invention
The present invention relates to an image capturing apparatus.
Description of the Related Art
In recent years, as an automatic focus detection technique, there is a technique called an on-imaging plane phase difference method. In this technique, photoelectric conversion is performed on a pair of subject images formed by light beams that have passed through two different areas (hereinafter, referred to as pupil areas) in an exit pupil of an imaging optical system, using a plurality of pixels for focus detection provided on an image sensor, in order to obtain a pair of picture signals from these pixels. A relative position shift amount (hereinafter, referred to as an image shift value) between these picture signals is then calculated by calculating the correlation of this pair of picture signals, and a focus shift amount (hereinafter, referred to as a defocus amount) indicating the focus state of the imaging optical system is calculated from this image shift value.
For example, in Japanese Patent Laid-Open No. 2013-236362, a configuration is adopted in which one microlens having a converging action and two photodiodes (hereinafter, referred to as PDs) are provided for one focus detection pixel, and these two PDs receive light beams from the two pupil areas. The above-described pair of picture signals can be obtained by providing a plurality of the above focus detection pixels in the image sensor.
In addition, in Japanese Patent Laid-Open No. 2012-80195, two focus detection pixel groups that are different in the direction in which an opening of a wiring layer provided in front of the PDs is shifted relative to the pixel center are provided in the image sensor. These two focus detection pixel groups receive light beams from the two pupil areas, and thereby a pair of picture signals can be obtained.
Incidentally, output signals from focus detection pixels used in an imaging plane phase difference detection method are used for focus detection, and thus high resolution is also demanded when A/D conversion is performed. However, there is a possibility that, if the resolution during A/D conversion is lowered in order to speed up A/D conversion, the resolution of the output signals of the focus detection pixels will be lowered, and as a result, the defocus amount detection accuracy will decrease.